As mobile devices have been increasingly developed, and the demand for such mobile devices has increased, the demand for batteries has also sharply increased as an energy source for the mobile devices. Also, much research on batteries satisfying various needs has been carried out.
In terms of the shape of batteries, the demand for prismatic secondary batteries or pouch-shaped secondary batteries, which are thin enough to be applied to products, such as mobile phones, is very high. In terms of the material for batteries, on the other hand, the demand for lithium secondary batteries, such as lithium ion batteries and lithium ion polymer batteries, having high energy density, high discharge voltage, and high output stability, is very high.
Furthermore, secondary batteries may be classified based on the construction of an electrode assembly having a cathode/separator/anode structure. For example, the electrode assembly may be constructed in a jelly-roll (winding) type structure in which long-sheet type cathodes and long-sheet type anodes are wound while separators are disposed respectively between the cathodes and the anodes, a stacking type structure in which pluralities of cathodes and anodes having a predetermined size are successively stacked while separators are disposed respectively between the cathodes and the anodes, or a stacking/folding type structure in which pluralities of cathodes and anodes having a predetermined size are successively stacked while separators are disposed respectively between the cathodes and the anodes to constitute a bi-cell or a full-cell, and then the bi-cell or the full-cell is wound.
FIG. 1 is a side view typically illustrating the general structure of a conventional representative stacking type electrode assembly.
Referring to FIG. 1, the stacking type electrode assembly 10 is constructed in a structure in which cathodes 20, each of which has a cathode active material 22 applied to the opposite major surfaces of a cathode current collector 21, and anodes 30, each of which has an anode active material 32 applied to the opposite major surfaces of an anode current collector 31, are sequentially stacked while separators 70 are disposed respectively between the cathodes 20 and the anodes 30.
From one-side ends of the cathode current collectors 21 and the anode current collectors 31 protrude pluralities of cathode tabs 41 and anode tabs 51, to which an active material is not applied, such that the cathode tabs 41 and the anode tabs 51 are electrically connected to a cathode lead 60 and an anode lead (not shown) constituting electrode terminals of a battery. The cathode tabs 41 and the anode tabs 51 are joined in a concentrated state, and are then connected to the cathode lead 60 and the anode lead, respectively. This structure is more clearly illustrated in FIGS. 2 and 3, which are partially enlarged views typically illustrating the joint portion between the cathode tabs and the cathode lead. FIGS. 2 and 3 illustrate only the joint portion between the cathode tabs and the cathode lead for convenience of description, although this structure is also applied to the joint portion between the anode tabs and the anode lead.
Referring to these drawings, the cathode tabs 40 are brought into tight contact with each other in the direction indicated by an arrow, and are connected to the cathode lead 60. The cathode lead 60 is normally joined to the cathode tabs by welding. The cathode lead 60 may be joined to the cathode tabs while the cathode lead 60 is located at the top of the uppermost cathode tab 41, as shown in FIG. 2. Alternatively, the cathode lead 60 may be joined to the cathode tabs while the cathode lead 60 is located at the bottom of the lowermost cathode tab 42, as shown in FIG. 3.
Due to this joint structure, however, the resistance difference between the electrodes with respect to each electrode lead may occur in the electrode assembly. Specifically, the electrode resistance of the electrode tab at the shortest distance from the electrode lead is different from that of the electrode tab at the longest distance from the electrode lead. In a middle- or large-sized battery pack including the electrode assembly with the above-stated construction, large-capacity electricity is charged and discharged. Consequently, the electrodes may be nonuniformly operated or deteriorated, due to the resistance difference between the electrodes, which may reduce the life span of the battery.
Also, when the electrode tabs are joined to the electrode lead in the above-described structure, a welding process for the joining the electrode tabs and the electrode lead is performed only in one direction, with the result that the joint force between the electrode tabs and the electrode lead may be lowered.
Consequently, there is a high necessity for an electrode assembly having an improved structure in which the joint force between the electrode tabs and the electrode lead is increased while the resistance difference between the electrodes is minimized.